qemu-char: socket backend: disconnect on write error

qemu-char: socket backend: disconnect on write error

Socket backend read handler should normally perform a disconnect, however
the read handler may not get a chance to run if the frontend is not ready
(qemu_chr_be_can_write() == 0).

This means that in virtio-serial frontend case if
 - the host has disconnected (giving EPIPE on socket write)
 - and the guest has disconnected (-> frontend not ready -> backend
   will not read)
 - and there is still data (frontend->backend) to flush (has to be a really
   tricky timing but nevertheless, we have observed the case in production)

This results in virtio-serial trying to flush this data continiously forming
a busy loop.

Solution: react on write error in the socket write handler.
errno is not reliable after qio_channel_writev_full(), so we may not get
the exact EPIPE, so disconnect on any error but QIO_CHANNEL_ERR_BLOCK which
io_channel_send_full() converts to errno EAGAIN.
We must not disconnect right away though, there still may be data to read
(see 4bf1cb0).

Signed-off-by: Anton Nefedov <[email protected]>
Signed-off-by: Denis V. Lunev <[email protected]>
CC: Paolo Bonzini <[email protected]>
CC: Daniel P. Berrange <[email protected]>
CC: Marc-André Lureau <[email protected]>
Message-Id: <1486045589[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>